The success of X-ray computed tomography has encouraged the proposal of other medical imaging techniques not fraught with the dangers of X-rays. The use of low frequency electric currents has been suggested although no practical results have been published, but some literature exists on methods which involve measurements of impedance between electrodes on the surface of the body and propose methods for reconstruction of spatial impedance variations, for example, "An Impedance Camera for Spatially Specific Measurements of the Thorax" by R. P. Henderson and J. B. Webster (I.E.E.E. Trans on Biomed. Eng. Vol. 25, pp 250-254, 1978), "An Impedance Camera: A System for Determining the Spatial Variation of Electrical Conductivity" by R. J. Lytle and K. A. Dines (Lawrence Livermore Lab. Rep. U.C.R.L. 52413, 1978) and "Reconstruction of Spatial Resistivity Distribution of Conducting Objects from External Resistance Measurements by H. Schomberg (Philips GmbH, Hamburg, Mn MS-H, 1908V/78, 1978). Other examples include "Electrical Impedance Imaging of the Thorax" by Y. Kim, J. G. Webster and W. J. Tomkins (Jn. of Microwave power, 18(3), 245-257, 1983): "Fundamental study on electrical impedance CT algorithm utilising sensitivity theorem on impedance plethysmography", by K. Nakayama, W. Yagi and S. Yagi (Proc. of Vth (Int. Conf. on Electric Bio Impedance), Tokyo, 99-102, 1981): "A fundamental study of an electrical impedance CT algorithm", by K. Sakamoto and H. Kanai. (Proc. of VIth ICEBI, Zadar, Yugoslavia, 349-352, 1983): and "Methods and feasibility of estimating impedance distribution in the human torso", by Y. Yamashita and T. Takahashi. (Proc. of Vth ICEBI, Tokyo, 87-90, 1981). A current review is given by D. C. Barber and B. H. Brown (Applied Potential Tomography, Jn. of Physics E., 17,723-733, 1984). However, the resolution problem is much greater than that involved in X-ray computed tomography because the current flow in tissue is not confined to the direct path between a pair of electrodes. L. R. Price has suggested in "Imaging of the Electrical Conductivity and Permittivity Inside a Patient: A New Computed Tomography (CT) Technique" (Proc. Soc. Photo-Opt. Instrum. Eng. (USA), Vol. 206, pp 115-119, 1979) and "Electrical Impedance Computed Tomography (ICT): A New CT Imaging Technique" (I.E.E.E. Trans. Nucl. Sci., Vol. NS-26, pp 2736-2739, 1979) a method which forces a sinusoidal spatial potential function on the conducting medium such that the current paths are parallel streamlines. The ratio of current to applied voltage is thus dependent on the line integral of conductivity and so standard tomographic reconstruction procedures can be used. However, R. H. T. Bates, G. C. McKinnon and A. D. Seager have shown in "A Limitation on Systems for Imaging Electrical Conductivity Distributions" (I.E.E.E. Trans. on Biomed. Eng., Vol. 27, pp 418-420, 1980) that it is impossible to uniquely reconstruct images in this way unless ambiguities can be resolved by making extensive sets of measurements.
A major problem with any practical tissue resistance imaging method lies in the electrodes used to make contact. Electrode impedance is significant in comparison with tissue resistance at frequencies less than 100 kHz and operation at higher frequencies is extremely difficult because capacitive currents become significant. The impedance changes to be expected due to the spatial distribution of tissue resistivity are small and it is certainly necessary to be able to make measurements to an accuracy better than 1%. This is unlikely to be possible if electrode impedance is indistinguishable from tissue resistance.